grouping some columns in date fields in pivot table

I have huge amount of data (about 6400000 rows) in Access 2007 format.
some fields (say Transdate) are date fields which have dates ranging from 1.3.2011 to 31.7.2012.
one field is number format (let say 'Amt')
one other field is for some identification (say 'grantno') having values (in text format) from 001, 002, ...., 999.
For a pivot table/chart i have created a pivot table in MS excel 2007 with link to that data in excel
In pivot table i have created rows with grantno, columns with transdate and values with Amt.
After that I have grouped the Transdate in Month and Years format - resulting in total 16 columns, 999 rows.

My problem is as under: I want only 12 columns with grouping as follows "Apr-11, may-11, .... Mar-12 (March 12 having total of March 12 to July 12)

posting a book with some test data with 'data' and 'pivot' sheets